Everything You Wanted to Know About Rear Suspension GMC Motorhome International 2014 Spring Convention Montgomery, Texas 1. Jacking A. Floor Jack* 1. Use bogie arm plate (vendor supplied) B. GMC Jack & Chain* 1. Follow instructions in Owner s Manual C. Jack Pads* 1. Use bottle jack 2 ton or greater 2. Place leaf spring (partial) under control arms *Always block frame after raising, NEVER rely on jacks holding weight 2. Air Bag(s) A. Rotate every two (2) years prevents hot dogging 1. Loosen nuts when bag(s) full of air 2. Raise bogie taking weight off bag(s) 3. Disconnect air line 4. Rotate bag(s) 180 degrees 5. Tighten nuts, do not over torque front and rear nuts after turning or replacing air bag(s) 6. Reconnect air line 7. Inflate bag(s) 8. Lower bogie B. Check For: 1. Aging a. cracks & tears* b. exposed thread and broken thread strands *Missing black rubber coating exposing threads does not indicate bag failure. 2. Leaks a. at O ring (front of air bag) use liquid soap solution in spray bottle, spray and look for bubbles b. air line fittings plastic more prone to failure, use brass if replacing c. stones or small debris between cone and bag after marketing deflectors (check with vendors) C. If Bag Breaks 1. Use threaded rod, 3/4 x 18 and four (4) nuts 2. Wooden block 13 1/2 long (insert between bogie air bag brackets) 3. Regussa a. cut bag and insert casting D. Maximum Bag Pressure 120 lbs. PSI, Minimum Pressure for Travel 60 lbs. PSI 1. Compressor will normally come on at 90 lbs. PSI when in travel mode a. Design I & II (single tank) maximum pressure 120 lbs. PSI minimum pressure 90 lbs. PSI
Everything You Wanted to Know About Rear Suspension Page 2 of 5 E. Install Shut Off Valves (air bag - vendor supplied) 1. For parking (helps prevent leak down) 2. Air line falilure a. have a way of maintaining air in bag(s) for safe travel 3. Side air line can be added for easy inflation F. Cones 1. Make sure cones are alumnimum, not plastic a. plastic cones can shatter 2. Keep cone clean and free from dirt and small stones G. New Style Air Bags 1. Floating two (2) bag system (one side) four (4) bags total 2. Fixed four (4) bag 3. Level System A. Check Height 1. Front: 13 1/8 plus or minus 1/4 a. measure rom elongated hole on front frame (measurement is with bias ply, 16 1/2 tires)* 2. Back 11 11/16 plus or minus 1/4 a. measure elongated hole of rear frame (measurement is with bias ply, 16 1/2 tires)* *measurement different with radial ply ties, with 16 wheels B. To Adjust Height 1. Rear a. loosen control, move up or down, tighten, re-check height if adjustments cannot be made, check arm linkage between control leveler and bogie arm C. Proper Use of Controls 1. Manual 1973 to 1976 (Design I) Travel: when coach is in motion Hold: to maintain position when parked Raise: to raise right or left side Lower: to lower right or left side 2. Electro Level I & II 1976 to 1978* Travel: auto for auto leveling, then hold when coach is in motion* Raise: to raise right or left side Lower: to lower right or left side *instructions changed to read same as Design I 3. Either system can raise or lower the rear of the vehicle (right or left side) approximately 4 (single air bag system only)
Everything You Wanted to Know About Rear Suspension Page 3 of 5 4. Wheel Bearings Total Eight (four inner 4 outer 4 seals) A. Check Yearly 1. Re-pack at 24,000 miles a. check for wear or heat b. check inside of rear wheels for thrown grease IMPORTANT! Caution is to be advised when changing real seal due to design change in the part number (89745). Improper seal will cause bearing failure in as little as 1,000 miles B. Torque to Specifications 1. 25 foot pounds, back off 1/2 turn, then finger tight a. should always be a little play in wheel, top to bottom CAUTION! If no play is present, over tighting can cause bearings to heat and cause bearing failure. WARNING! b. purchase a heat sensing gun, check hub heat at rest stops, all hubs should be reading equal heat if a hub is found hot (to hot to keep a finger on do not proceed until problem has been found. NEVER OVER TORQUE 5. Bogie Pins four (4) A. Grease approximately every 2,000 miles 1. Raise bogie until wheels are off the ground 2. Drop air in bag(s) 3. Use shovel under wheel, moving up and down while greasing (seond person a big help) 4. Make sure grease exits from front and rear of pin 5. With wheels elevated check to make sure there is no play on control arm (pushing and pulling front edge of tire should have no play) B. Removing Old Grease from Pins 1. Remove zerk fitting 2. Insert straw 5/16 diameter approximately 9 long (Arby s work great) a. push in until approximately 1/2 sticks out, twist, pinch end, remove straw 3. Re-greasing a. use quality hi temp grease, (such as Mobil 1 synthetic or equal) b. 1973 to 1974, 1 1/4 pins, one (1) inner grease slot follow procedure A. c. 1975 to 1978, 1 1/2 pins, two (2) inner grease slots follow procedure A or use lube tube (vendor supplied), remove zerk, insert tube (after grease has been removed) apply grease, grease should come out of rear only, remove lube tube, re-install original zerk, apply grease, grease should appear in front of pin.
Everything You Wanted to Know About Rear Suspension Page 4 of 5 6. 4 Wheel & 6 Wheel Alignment Check A. Make sure bogie arm is straight and pin is not worn 1. Jack side, remove wheels, put straight edge 1 1/2 x 1 1/2 x 48 aluminum 90 angle on wheel hubs a. hubs should not have gaps wheel to wheel HUB HUB X X X X X No Gaps Straight Edge If so, control arm can be bent back into position using an arm pusher* drawings available. B. With straight edge on hub, face laser level on aluminum angle, shoot forward to front wheel, measure with tape (laser dot to face front hub, record, measure opposite side using smae method, record, recorded measurements should be within plus or minus 1/4 of each other *According to Cinnabar Engineering, the only way to truely get a 6 wheel alignment is to use slip plates on all 6 wheels, other methods can be used, always seek propfessional help C. If the frame is not bent, there is no camber or caster adjustment. Toe in and toe out are done by shimming the rear bogie (providing the control arm is not bent, and pins are not worn) 7. Brakes A. Check Yearly 1. Show wear 2. Drum wear a. dust edge breaking off b. weights missing 3. Cylinders leaking 4. Brake lines and hoses a. worn or bulged 5. Adjustments a. for slight drag b. re-tensioning parking brake 6. Bleeding a. gravity b. pressure lock out equalizing valve c. vacumn pumping lock out equalizing valve
Everything You Wanted to Know About Rear Suspension Page 5 of 5 8. Shock Absorbers A. Check for leaking B. Check for bent or broken ears and rods, or loss of retaining nuts 9. Every Trip A. Check for foreign material in air bag cones B. Ride height C. Feel hubs, if you cannot keep you fingers on hub due to heat DO NOT PROCEED! Wheel Bearing Failure.
Designer General Motors Leigh Harrison Jim Kanomata Zeb Frady Alex Sirum Dan Hensley Jim Bounds Silver Motor Coach System name OEM Four Bag Quadra Bag Southland Hensley Big Ole Bag (BOB) Silvertone Year introduced 1973 1995 2011 not yet! 1991, 2011 GMC Bags per side 1 2 2 1 2 1 1 1 Bag manufacturer Firestone Firestone Firestone Firestone Firestone Bag part number 8301 W01-358-3400 W01-358-7325 Leigh Harrison, generally orderable Applied GMC, COOP, Alex Sirum, Grandview, MGM, generally orderable Southland Bag vendor, USA, retail none Motorhome Center Applied GMC Bag price w/o shipping, each (date) n/a $100 (May 2011) $100 (May 2011) $300 (May 2011) $400 (May 2011) Max height under bogie Pressure at max height Min height under bogie Bag pressure at normal ride height and weight, psig 80-90 100-110 80-90 90 90-100 125 Leveling range, inches: +up, -down full: +4, -4 reduced: +3, -4 full: +4, -4 reduced: +3, -4 greatly reduced Southland Motorhome Center Alex Sirum GMC Motorhomes Dan Hensley Applied GMC System vendor none Leigh Harrison Applied GMC GMC COOP, Alex Sirum, Grandview, System vendors, other MGM System price, both sides (date) n/a $1075 (May 2011) $1600 (May 2011) $950 (May 2011) $600 (May 2011) $800 (May 2011) System weight 57 lbs Shipping price, ~2000mi USA n/a $50 Time to install, both sides n/a 4-6hrs 4-6hrs 1hr 1hr Ease of installation n/a straightforward but some difficult to access and/or high torque work Systems sold (date) 12,921 (1979) 200+ (Jun 2011) 12 (Jun 2011) 5 (May 2011) Design features Reviews, fans Reviews, skeptics should survive loss of single bag per side (2x load on remaining bag and tire!) improved handling (esp for towing) good protection of hoses and bags from road debris firmer ride than OEM isolated report of damage to center support under extreme road dynamics straightforward but some difficult to access and/or high torque work easy easy easy should survive loss of single bag per side (2x load on remaining bag and tire!) improved handling (esp for towing) firmer ride than OEM rides like OEM rides like OEM can travel with bags deflated, ~2" clearance between tires and wheel wells question about side-load stresses
Designer General Motors Leigh Harrison Jim Kanomata Zeb Frady Alex Sirum Dan Hensley Jim Bounds Silver Motor Coach Notes bags no longer manufactured some NOS bags are still available but cost prohibitive (e.g. Cinnabar, $700/ea?) adapted from Harrison design with many updates (details will be added soon). cone design patented Southland is only US distributor for bag, but it is said to be common in international trucking use site says "new style" air bag system in stock, no link or info bag/cone system original on Silver MC vehicles in 1991 repurposed for TZEs in 2011 future availability and current bag age uncertain More information Leigh Harrison product page air bag data sheet 3400 (PDF) bellows data sheet 224 (PDF) Applied GMC product page installation instructions (PDF) ait bag data sheet 7325 (PDF) bellows data sheet 26 (PDF) Southland Motorhome Center product page GMC ES presentation (PDF) installation video
Brakes Available Today Brake Type Status Features 78 Cadillac OEM Available in Junkyards Discs Only -- Middle and/or Rear -- Large Calipers w/ or w/o Parking TSM (The Street Machine) Delivering from TSM Discs Only -- Middle and/or Rear -- Large Calipers w/ or w/o Parking Leigh Harrison Delivering from Iden Corp Discs Only -- Front, Middle and/or Rear -- Large Calipers -- Extra Large Rotors Applied GMC Delivering from Applied GMC Discs Only -- Middle and/or Rear -- Large Calipers Reaction Arm System Status Features Norm Jestico Reaction Arm Prototyped by Deceased Norm Discs Middle and Rear -- Large Calipers w/ or w/o Parking Sway Bars Reaction Arms Give Some Track Control Chuck Aulger Reaction Arm Delivering from Applied GMC Discs Middle and Rear -- Large Calipers w/ or w/o Parking Sway Bars Reaction Arms Give Some Track Control Chuck Aulger Reaction Arm Delivering from Applied GMC Discs Middle and Rear -- Large Calipers w/ or w/o Parking Reaction Arms -- No Track Guides Manny Brakes Delivering from Manny's Trannys Discs Middle and Rear -- Small Calipers w/ or w/o Parking Optional Track Guides Tom Pryor Reaction Arm Delivering from Applied GMC Drums and/or Discs -- Middle and Rear Optional Straight Track Guides